The primary aim of this pilot study is to characterize the proactive and reactive locomotor response of Veterans with upper limb loss (ULL) to a trip during walking. Our recent VA-funded (RX001388, RX001322) investigations have suggested that persons with ULL experience a high prevalence of falls and demonstrate postural control mechanisms that may impair stability. Specifically, nearly half of individuals with ULL at or proximal to the wrist level experience at least one fall per year and almost a third will experience two or more falls. Further, use of a prosthesis increases the likelihood of falling by six times, 25% of reported falls resulted from tripping, and nearly a third of individuals who experience a fall suffer a fall-related injury. Falls can have considerable economic burden on the VHA and lead to long-term diminished quality of life. Our biomechanical studies suggest that persons with unilateral ULL display greater postural sway during standing than able-bodied individuals which increases when wearing a prosthesis, and right/left asymmetry in locomotor stability dynamics that may [increase the risk of falling toward the impaired limb side and during sound limb side strides]. These findings emphasize the need for additional research to better understand the mechanisms Veterans with ULL use to control balance and how [wearing a] prosthesis [affects] these strategies. As our previous research was concerned with steady- state characterization of postural control, we now plan to build on this work by studying the effects of ULL and [wearing a] prosthesis on locomotor stability when responding to a trip disturbance during walking. In this context, locomotor stability is defined as the ability to recover from a perturbation and return to steady-state gait. We will address the study aims by analyzing trip-induced proactive and reactive locomotor strategy differences in two study comparisons: 1) Veterans with unilateral transradial level ULL against matched able-bodied controls [(with and without one arm bound)], and 2) Veterans with unilateral transradial ULL when wearing their customary prosthesis against not wearing their prosthesis. Controlled, yet unexpected, simulated trips will be delivered through our custom-built treadmill which permits programmable belt velocity disturbances and allows participants to continue walking following recovery. We will characterize the proactive and reactive locomotor [stability mechanisms] through a set of biomechanical (spatiotemporal, angular momentum, arm and trunk kinematics, recovery step length and time) and electromyography (upper and lower limb muscle activation timing and effort) variables. Biomechanical variables will be quantified using an optical motion capture system, and electromyography will be collected with a wireless sensor system. All data will be synchronized with each other and a time pulse denoting the onset of the treadmill perturbation. We expect that Veterans with ULL will demonstrate [altered] locomotor stability [mechanisms] compared to controls, and [these differences will exist between wearing and not wearing] their customary prosthesis. Results from this study will help us characterize the underlying mechanisms of locomotor stability in Veterans with ULL and identify the factors associated with their increased prevalence of trip-related falls. Such knowledge is a critical first step to addressing this public health problem through stability-targeted rehabilitation interventions aimed at reducing falls, fall-related injuries, and associated VHA costs in this Veteran patient group. We will use the outcomes from this pilot study to guide future VA Merit Award proposals to [develop and] assess [physical training] intervention methods and [wearable and prosthetic technology to improve stability in Veterans with ULL]. The VHA is an ideal venue to pursue this work as one of its main priorities is to elevate the standard-of-care for Veterans with limb loss.